In tire retreading operations, it is highly desirable, if not essential to make a thorough inspection of a tire casing before subjecting it to the retreading process. If the tire casing is retreaded with some latent defect, it must later be discarded when the defect is discovered and, until discovered, the defect may constitute a serious safety hazard.
Heretofore, efforts have been made to provide adequate non-destructive tire inspection apparatus utilizing vibration techniques, X-rays or ultrasonic energy as the media for analyzing the tire structure. Recent examples of a tire inspection apparatus utilizing ultrasonic energy are found in U.S. Pat. Nos. 4,266,428, 4,275,589 and 4,365,514. However, such prior inspection systems and apparatus have not provided a level of performance, i.e. speed, accuracy and reliability, that has been sufficient to gain wide acceptance in the tire recapping industry. In prior tire inspection apparatus, as described in the aforementioned patents, the ultrasonic energy is provided in relatively long bursts inside an inflated tire being inspected to acoustically illuminate the entire inside surface of the tire. As the tire was rotated on the apparatus, the acoustic energy passing through the tire was sensed by plural acoustic receiving transducers arranged in a fixed preset array about the outer tire walls. Variations in received signals by the receiving transducers of the array were presumed to provide a basis for detecting flaws or leaks in the tire carcass. However, in this prior art device certain difficulties arose such as the leakage of relatively high levels of acoustic energy within the inflated tire which often interfered with the reception of clear signals by the arranged receiving transducers. Thus, the accuracy and dependability of the inspection system with regard to the consistent identification of the anomalies was often unacceptable and in need of improvement.
To overcome the deficiencies of prior tire inspection systems, the present system was conceived utilizing high frequency acoustics in a through transmission arrangement comprising the combination of a transmitting pulse generator, a transmitting transducer for producing collimated bursts of ultrasonic energy, a separate receiving transducer, so placed to receive the acoustic energy transmitted through the material, an acoustic coupling medium on both sides of the material and amplifiers, processors and indicators or displays. However, the inspection of tires by such acoustic means presented particular problems due to the inherent factors such as: (1) the requirement that the coupling medium between transducers and the tire material must be air; (2) the tire has an irregular shape; (3) the tire inspection apparatus for tires is commonly located in areas where ambient noise levels are high which affects the evaluation of test signal data; and (4) the acoustic transmitter and receiver must be coordinated to move in concert to assure that the transmitted collimated energy is properly received for each burst.
Although it is more practical to use air as a coupling medium for tire inspection, significant problems exist with its use, such as: (1) air coupled transmitting transducers are of fairly limited output intensity; (2) a large acoustic impedance mismatch exists between air and any solid material being inspected; (3) a large refraction of the acoustic energy occurs at the entry and exit surface when the acoustic "beam" is not normal to these surfaces. (The index of refraction between air and rubber is approximately 5); and (4) a significant ambient noise level exists in most industrial environments in which a system is operating.
With regard to the tire shape, the refraction effect in an air medium becomes a serious problem because: (1) the inner and outer surfaces are not coaxial; (2) the outer surface is fairly flat on the running surface and cylindrical on the side walls; and (3) the inner surface may have a large degree of eccentricity.
A general object of this invention is therefore to provide an improved tire inspection system that overcomes the aforesaid problems and will: (1) utilize a highly directional transmitting transducer to efficiently "illuminate" an area on the inner surface of the tire and be maintained near normal to that area; (2) utilize a highly directional receiving transducer that is relatively insensitive to ambient noise, and maintain it in a position that is near normal to the outer surface of the tire; (3) control the relative positions of the transmitter and receiver transducers so that the receiver is always aligned in the area of the exiting refracted acoustic energy; (4) minimize the effect of varying ambient noise on the automatic gain controls within the signal processor.
Other objects of this invention are to provide a tire inspection apparatus that: (1) is relatively easy to operate by persons having an ordinary level of skill; (2) is highly accurate in detecting tire defects: (3) provides a visual mark at the location on the tire for each defect detected; (4) provides a displayed readout of the number of inspection pulses and the number of defects detected; and (5) is reliable and relatively easy to service and maintain.